An injured boy recovering in a hospital in Dahuk, 430 kilometers (260 miles) northwest of Baghdad

Rescuers tore with their bare hands at the wreckage of mud-brick houses in the Sinjar district of northern Iraq yesterday where five vehicle-born bombs have killed at least 200 people and injured 300.

The casualties may rise to make the atrocity the worst single bombing of a civilian target in Iraq in the past four years. All the victims were Yazidis, members of a pre-Islamic sect, many of whom live in this part of northern Iraq.

"We are digging with our hands and shovels because we can't use cranes because many of the houses were built of clay," said Dakhil Kassim, mayor of the nearby town of Sinjar. "We are expecting to reach the final death toll tomorrow or the day after tomorrow as we are getting only pieces of bodies."

The loss of life is so high because the Yazidis are poor and live packed together in houses constructed of mud brick. These provide no protection against the force of a bomb blast. The most likely perpetrators are Sunni Arab Jihadi insurgents who see all those who do not belong to their own brand of Islam as deserving death.

The Islamic State of Iraq, an al-Qa'ida in Iraq umbrella organisation, distributed leaflets a week ago warning residents in the area that there was going to be an attack because Yazidis were "anti-Islamic".

Mr Kassim said that on Tuesday four trucks had entered the town of Qahataniya 70 miles west of the city of Mosul and detonated large bombs almost simultaneously. The US military say there were five bombs. A total curfew has been declared in the Sinjar area.

Conflict between Arab and Kurd, Sunni and non-Sunni has been rising sharply this year in northern Iraq. Arab-Kurdish friction has increased in and around the cities of Kirkuk and Mosul. In both places Arabs and Kurds are vying for control. In Mosul city, a total of 70,000 Kurds have fled persecution, according to Khasro Goran, the deputy governor.

Sectarian conflict between Yazidis and Muslims has escalated since a so-called "honour killing" earlier this year in which several thousand Yazidis stoned to death a Yazidi girl who had converted to Islam in order to marry her Muslim boyfriend. Her gruesome death was recorded on mobile phone cameras and later shown on websites. In revenge, 23 Yazidi textile workers were taken off a bus by gunmen and shot dead.

The attacks in Sinjar underline the inability of the government in Baghdad to control a series of very distinct battles for supremacy taking place in different parts of Iraq. This is not only between communities but also within them.

In the Shia city of Diwaniyah last Saturday, an expertly timed roadside bomb killed the governor, Khalil Jalil, and the provincial police chief, Maj-Gen Khalid Hassan. The assassinations may be part of a war for control of the province between the Mehdi Army militia of the radical Shia cleric Muqtada al-Sadr and Iraqi government forces who are loyal to the Supreme Islamic Iraqi Council, of which Mr Jalil was a senior member.

The intra-Shia conflict has little to do with the US and British occupation and is primarily over the control of jobs and local resources. In cities such as Basra, control of oil products and the port are particularly valuable.

The struggle for power in northern Iraq is likely to escalate sharply in coming months because a referendum is scheduled at the end of 2007 in which people in Kirkuk and Mosul will vote on whether or not to join the semi-independent Kurdistan Regional Government (KRG).

Although the timing of the referendum is written into the constitution, the government in Baghdad has been very slow in reversing Saddam Hussein's ethnic cleansing of Kurds and holding a census to determine who can vote.

The Kurds are growing impatient about holding the referendum but might agree to a short delay. They would expect the result of a vote to show Kirkuk city and the surrounding oilfields joining the KRG but Mosul city staying out. On the other hand, the Kurds would hope to take over areas around Mosul city such as Sinjar, the site of this week's bombing, because it is also their link to the large Kurdish minority in northern Syria. Militant Sunni areas such as Hawaijaqh in western Tameem province would probably secede.

The US military has suggested the bombers are operating more ruthlessly in northern Iraq because they can no longer operate in Baghdad because of the success of the American "surge". In reality, the number of car bombings in Baghdad in July was 5 per cent higher than last December and civilian casualties in explosions have increased by about the same percentage.